The year is 1340 and King Alphonse XI (Alfonso XI) reigns in Castile. Father Baltasar, the superior of the fictitious Monastery of St. James of Compostela, is speaking to Fernand, a novice who is about to take his vows and in whom Balthazar has placed all his hopes and expectations as his successor. Fernand confesses to him that he has fallen in love with a beautiful and mysterious woman whose praises he sings, and that he dreams of a life beyond the monastery walls. Balthazar warns him of the perils of the world, but Fernand has made up his mind to leave the monastery and there follows a tearful farewell between the two.
In a paradisiacal retreat on the Island of León near the Portuguese coast (where King Alphonse is supposedly in the final stages of establishing alliances against the Moors) a group of young ladies are happily singing and dancing. Fernand, blindfolded in order to be escorted to Leonor, arrives by boat. Her timely appearance gives way to the opportune amorous displays, nevertheless, Leonor refuses to reveal her true identity. Even so, Fernand asks her to marry him but she responds that it is impossible and he must not see her again, she then hands him a piece of paper. Inés announces the King’s arrival, and Fernando, left alone, reads with pleasure that he has been
named a captain.
The battle (the Battle of Salado) has been won against the Moors. King Alphonse, strolling through the gardens of Seville’s Alcazar, praises their beauty while commenting to Don Gaspar on the great victory achieved thanks to the bravery of Fernand whom he wishes to ennoble. Once alone, Alphonse expresses his intention of repudiating the queen to marry Leonor who then appears and reproaches him for her sad fate as his mistress, requesting his permission to leave for good. The King refuses to grant it and tells her to trust him. During the big victory celebration that is being held Don Gaspar enters and tells the King that Leonor has taken a lover. As proof he gives Alphonse a letter that has been intercepted. Under questioning by Alphonse, Leonor confesses her love for another but refuses
to give the King his name. They are interrupted by Balthazar who bursts in waving a papal bull in his hand that threatens the King with fire and brimstone and even excommunication if he doesn’t immediately renounce and banish Leonor. In the final Concertante the Kind responds with fury to the Church’s interference against his wishes and threatens revenge, Leonor and Inés display their fright of the anathema and their insecure situation, and Don Gaspar and the court implore that Leonor will be banished and that they have time to get out of the Alcazar before if falls on their heads as a consequence of Balthasar’s censure.
In one of the halls of the Alcazar Fernando excitedly awaits Leonor. Don Gaspar enters with the King who upon seeing Fernand offers to grant him a reward for his bravery in battle. Fernand requests the hand in marriage of his beloved, whom he points to as she so conveniently enters. The surprised King reacts by ordering the wedding to take place immediately. Leonor, believing that Fernand should learn the truth before the ceremony, sends Inés on her behalf to tell him that she is the King’s paramour. Leonor exits first but Inés, left alone, is detained on the King’s orders and cannot deliver the message.
The wedding ceremony begins and the King honours Fernand with a number of titles and a diamond necklace. Leonor is exultant because she believes that in spite of what Inés has told him he is willing to accept her. However the courtiers, lead by Don Gaspar, are convinced that he is aware of the King’s affair with Leonor and snub Fernand who in ignorance, reacts by challenging them to duels. When the swords are about to be raised Balthazar enters and clears up the misunderstanding. A horrified Fernand censures the King for his ignominious behaviour and rejects his gifts and titles, thereby transforming the courtiers’ disdain for him into admiration.
Disillusioned with the ways of the world, Fernand, back in the Monastery of St. James of Compostela, is about to take his final vows. Leonor appears disguised as a novice and after a grueling pilgrimage in search of Fernand faints when she hears him pronounce his vows. Fernand who has his misgivings about the decision he has taken goes out of the church and discovers Leonor there. They recognize each other and he asks her to go away. She tells him that she had been under the impression that he knew all about her and the King and asks his forgiveness. After some indecision, Fernand, who realizes that he’s falling in love with Leonor again, pardons
her and decides to go away with her. However Leonor wants only his forgiveness and once granted, after imploring God to save them both, serenely expires in his arms. Accompanied by the other monks Balthazar appears and once aware of who is lying there dead, tries to avoid a scandal by silencing Fernand and covering Leonor’s face with her hood. The superior implores all to pray for the novice who has just died.